Hydrophilicity Modification And Characterization Of Polypropylene

Polypropylene ( PP ) have good properties and wide-range applications, Since there are no polar groups on the polymer chain, its applications are limited. In order to overcome this problem and expand its applications, the hydrophilicity modification was necessary to PP. In this work, modification methods, including blending, plasma treatment, plasma polymerization and plasma-induced vapor phase modification, were employed. The effect of hydrophilic modification method on its hydrophilicity, chemical structure, surface morphology as well as the modification mechanism was discussed, respectively.Blending modification is a convenient and effective method. Hydrophilic PP pellets can be obtained by blending small amount of hydrophilic additives with PP. Comparing with unmodified PP, the addition of hydrophilic additives enhance the fluidity of PP melt but no obvious change was found in its rheological behavior. Therefore, the same spinning and extension processes to unmodified PP are still applicable to the modified PP. For example, its tensile strength is higher than 6.2cN/dtex. The hydrophilicity of PP pellet and fiber was evaluated by the contact angle (CA) of melt pressed chips from these materials. The CA of modified PP pellet and fiber decreased from 99°to 50°and 64°, respectively. The difference was probably due to the loss of hydrophilic additives to some extent during spinning. The decrease in CA of modified fiber indicated the blending modification can effectively improve its hydrophilicity. The surface morphology of PP fiber was observed by SEM. It was found that hydrophilic additives in the form of small domains dispersed in PP fiber uniformly. These domains were slightly extended along the fiber axis and the average width of domains was about 200 nm. Although modified PP fiber showed good dispersibility in water, the additives on fiber surface were easy to be washed out and decreased its hydrophilicity. It was also found that the hydrophilicity recovered over time likely due to the migration of additive from the bulk of modified PP fiber to the surface.Low-temperature plasma technique is a dry chemistry method. It can effectively improve the surface hydrophilicity of polymer materials without changing it bulk properties. In this work, a low-pressure glow discharge plasma treatment was given to PP film to improve its hydrophilicity. The optimum treatment conditions were explored for PP film by examining the effect of treatment time, power, gas flow rate, etc. It was found the CA of PP film under optimum treatment condition can decrease to 4°, which indicated that a very good hydrophilicity was obtained. However, the hydrophilicity tends to reduce over time.In order to overcome the ageing effect of merely plasma-treated properties, plasma polymerization and plasma-induced graft polymerization were applied to PP films, respectively. During plasma polymerization, the mixture of CH4 and O2 was chosen as plasma gas. The influence of CH4/O2 volume ratio and plasma parameters on hydrophilicity and ageing effect were studied. PP films modified by plasma polymerization showed good hydrophilicity. Its CA can decrease to 14°, which increased to 55°after 15 days. However, the CA of merely O2 plasma-treated PP film increased quickly from 4°to 72°only after 4 days. This result indicated the plasma polymerization can more effectively overcome ageing effect of improve hydrophilicity compared to the mere plasma treatment.A long-term hydrophilicity modification can be obtained through plasma-induced graft polymerization of acrylic acid (AAc) onto PP film in vapor phase. The grafting reaction time, temperature, pressure were studied to explore the optimum grafting condition. FTIR-ATR and SEM proved that AAc can be successfully grafted onto PP film within 10 minutes under an optimum condition. The CA of the grafted PP film was 32°, which remained quite well over time or even after a treatment in boiling water. The mechanism of plasma-induced graft polymerization in vapor phase was also discussed preliminarily. It was thought that this fast and effective method of graft polymerization in vapor phase could provide a strong experimental support to develop industrial applications in this field.